نبذة مختصرة : Objective. The aim of the present study was to evaluate transvaginal bleeding (TVB) as a risk factor for Rhesus isoimmunization during pregnancy, in order to optimize the application of Anti-D gammaglobulin in non-immunized pregnant women, as an alternative to the routine application of Anti-D at 28 weeks of gestation. Material and Methods. This case-control study was conducted from 1995 to 2001 at Mexico’s National Perinatology Institute. Cases (n=24) were non-immunized pregnant women who showed positive anti-D antibody seroconversion during pregnancy or during the early puerperium. Controls (n=24) were non-immunized pregnant women who enrolled after each case, with similar clinical characteristics but who had no anti-D antibody seroconversion during pregnancy. In all cases the newborns were Rh-positive. None of the patients received immunoprofilaxis at 28 weeks of gestation. The presence of TVB was recorded at any stage of pregnancy and before labor. Odds ratios with 95% confidence intervals were used to assess associations. Results. TVB was observed in 18/24 (75%) cases and in 5/24 (20%) controls. Preterm uterine contractions and threatened miscarriage were the most frequent causes of TVB. The presence of one TVB event during pregnancy increased 11.4 times (95% CI 2.9-44.0) the likelihood of Rhesus isoimmunization. TVB after 20 weeks of gestation increased the likelihood 5.0 times (95%CI 1.3-19.1). TVB before 20 weeks of gestation was not significantly associated with Rh isoimmunization (OR=7.6, 95%CI 0.8-69.5). Conclusions. Prophylaxis with
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